n/a
n/a
The present invention relates to fiber optic communications, and in particular to an system and method for spectrally conditioning the output of a fiber optic directly modulated laser transmitter to reduce the effects of dispersion and increase the span length between network elements.
The proliferation of computing and networkable devices has created a need for increased bandwidth between locations, whether those locations are local, regional, national, or international. A technology extremely well suited to supporting high data rates over long distances is fiber optic communications. Typically, a fiber optic communication link includes a fiber optic transmitting device such as a laser, a fiber optic cable span, and a light receiving element. Fiber optic transmitters and receivers are typically quite extensive. As such, there is a desire to be able to increase the span length, i.e. increase the distance between network end points. However, the adverse effects of noise, attenuation and dispersion limit the distance between network elements. This impact is particularly seen as transmission rates increase, because as transmission rates increase, the sensitivity of the system to noise and dispersion also increases, effectively further limiting the span length as data rates increase. It is therefore desirable to have a method and system which increases dispersion-limited distance and permits the use of less expensive lasers in long distances.
FIG. 1 is a graph generally showing light output as a function of current input into a laser. As shown in FIG. 1, there exists a knee 10 at which point the slope increases, i.e. light output increases at a greater rate for a given amount of current input into the laser than at points below the knee 10. It is therefore desired to operate a laser at a point just above knee 10 such that for a small amount of current, light output increases in an amount sufficient for a receiver to be able to detect a light existence, i.e. xe2x80x9c1 bitxe2x80x9d condition from a light off, i.e. xe2x80x9c0 bitxe2x80x9d condition. In operation, the light signal level for a xe2x80x9c0 bitxe2x80x9d is just above the knee and the light signal level for a xe2x80x9c1 bitxe2x80x9d is at the rated power output of the laser. The ratio of the xe2x80x9conxe2x80x9d to xe2x80x9coffxe2x80x9d light for a 1 and 0 is referred to as an extinction ratio. It is desired to have a large extinction ratio number.
For directly modulated (xe2x80x9cdirect modxe2x80x9d) lasers, operating the last above knee 10 reduces the optical noise and signal distortion. However, the trade-off is extinction ratio which shows up as a sensitivity penalty at the receiver. As such, there is a trade-off between the distortions and noise caused by the high extinction ratio at or below knee 10 and a low extinction ratio receiver penalty. Further, operating a direct mod laser below the knee 10 results in unwanted noise, referred to as chirp.
Section 2xe2x80x942 in FIG. 1 corresponds to knee region 12 and is shown in exploded view in FIG. 2. As shown in FIG. 2, knee region 12 is sub-divided into six sub-regions labeled a, b, c, d, e, and f, respectively. As is shown in FIG. 2, the slope of each successive sub-region increases. The relationship between the increasing slope and sub-regions a-f is explained with reference to FIG. 3. FIG. 3 is a chart showing optical spectrum emitted for each bias environment depicted in FIG. 2. As shown in FIG. 3, the laser, when operating in sub segment f, has a high intensity about the laser wavelength p. This high intensity allows the receiver to clearly discern that a xe2x80x9c1xe2x80x9d has been transmitted. As sub-regions along the knee are traversed, the intensity decreases, and the spectrum of light emitted by the laser increases. The result is a dispersion in the energy transmitted by the transmitting laser as detected by the receiver, and further results in unwanted noise, i.e. spectral content far removed from point p. The resulting impact is that this unwanted noise, i.e. chirp, adversely impacts the transmission capabilities of the system.
Another factor which limits span distance and which is exacerbated by the existence of chirp is fiber dispersion. The wider the spectral output of the laser, the more differentiation in the dispersion of the fiber at the receiver. In other words, the wider the spectrum at the transmitting end, the more penalty is paid at the receiving end. As shown in FIG. 4, there are three main types of dispersion known to those of skill in the art. Multi-path (multi-modal) dispersion is illustrated in fiber 14. Chromatic dispersion is illustrated in fiber 16 and polarization mode dispersion is shown in fiber 18. Multi-path dispersion and polarization mode dispersion are not directly relevant to the subject invention and their discussion is therefore omitted.
Chromatic dispersion, shown in fiber 16, results from a characteristic in which different wavelengths of light travel at different velocities in a fiber optic cable. As a result, a wider spectral content results in a wider differentiation in arrival times of the light pulses, thereby causing intersymbol interference. For example, referring to fiber 16, a pulse transmitted at a given point which has a non-narrow spectral content results in a portion of the spectral content arriving at point x in a given time t, while other spectral portions of the same transmission only travel to point y in time t.
Eye diagrams 20a, 20b, and 20c show the adverse effects of the various types of distortion along a fiber optic cable. These effects are shown by the decrease in eye 22a, 22b, and 22c sizes along the distance of the fiber. The wider the eye, the easier it is for a receiver to detect the absence or presence of a bit. However, the longer the fiber, the more dispersion and the narrower the eye. Further, the shorter the bit period, the faster the effect impacts the receiver. As such, reducing the effects of dispersion along a fiber results in a wider eye, making reception easier. One way to accomplish this is by tightly controlling the transmission to, for example, reduce the effects of dispersion more effectively limiting the light spectrum transmitted by the laser. This can be accomplished by controlling chirp.
Chirp controlling technologies are expensive and are presently addressed by electrical regeneration, dispersion-compensating modules, or by generating a clean pulse shape. Regeneration is inefficient, because it requires the addition of network components due to limiting span length. Dispersion-compensating modules waste optical power and often require the addition of optical amplifiers. A clean pulse shape can be generated, thereby controlling chip by using externally-modulated lasers. However, externally-modulated lasers are larger in size than their directly-modulated counterparts and are significantly more expensive. It is desirable to have an arrangement which controls chirp, thereby reducing the effects of fiber dispersion in a manner which allows the use of an inexpensive directly-modulated laser without the need for additional external components such as dispersion-compensating modules, light-regenerating devices, and the like.
Standards such as those issued by the International Telecommunications Union (xe2x80x9cITUxe2x80x9d) specify a grid which includes standard light wavelengths for different transmission bit rates. The grid sets forth center optical frequencies for a band pass filter mask inside of which the transmission frequencies must reside. This band pass filter mask becomes particularly important due to frequency drift experienced by lasers as they age as well as a change in the characteristics in filter/wavelength division multiplexing (xe2x80x9cWDMxe2x80x9d) coupling devices used to facilitate fiber optic communications.
Because the wavelength of light emitted by a laser is a function of the temperature of the laser, prior art devices have attempted to control the emitted light wavelength by monitoring the temperature of the laser using a device such as thermistor and heating or cooling the laser, as necessary, to attempt to maintain a fixed frequency. In this manner, manufacturers have attempted to provide fiber optic transmission systems which remain within the ITU grid during the operating life of the laser. These methods have finite precision and do not take into account the aging characteristics of the filter. Accordingly, it is desirable to have a method and system which provides for transmission of a specific wavelength in order to comply with known standards in a manner which is accurate despite the changes in performance characteristics of the filter.
The present invention advantageously provides a method and system which reduces the spectral output at a transmitter so that only the required spectrum is transmitted into the fiber optic cable. In order to reduce the spectral output, the present invention reduces chirp and locks the wavelength output of the emitted transmission laser light with respect to the filter edge. The effect is to minimize the dispersion penalty and allow an increase in network span. In addition, the present invention advantageously allows the use of inexpensive direct-mod lasers. Further, laser temperature control is not based on thermally-sensing laser temperature, but is instead based on the wavelength of the emitted transmission laser light.
According to an aspect of the present invention, an optical spectral conditioning system has an optical filter and a laser. The optical filter has an input for receiving a first optical signal, an output which provides a filtered optical signal and a filter profile. The filter profile includes a high wavelength skirt at an upper wavelength region of the filter profile. The laser is optically coupled to the optical filter input and emits the first optical signal. The laser is controllable to emit the first optical signal at a wavelength proximate to the optical filter high wavelength skirt.
According to another aspect, the present invention provides a method for spectrally conditioning an optical signal emitted by a laser and filtered by an optical filter optically coupled to the laser and having a characteristic filter profile including a high wavelength skirt at an upper wavelength region of the filter profile. The method includes controlling the laser to emit an optical signal at a wavelength proximate to the optical filter high wavelength skirt.
According to still another aspect, the present invention provides an optical spectral conditioning system, in which an optical filter has an input for receiving an emitted optical signal, an output providing a filtered optical signal and a filter profile. The filter profile includes a high wavelength skirt at an upper wavelength region of the filter profile. A directly modulated laser is optically coupled to the optical filter input and emits the emitted optical signal. The directly modulated laser is operable to emit the emitted optical signal at a wavelength proximate to the optical filter high wavelength skirt.